HeCd Laser Characteristics and Safety

The Helium-Cadmium (HeCd) laser is one of a class of gas lasers using
helium in conjunction with a metal which vaporizes at a relatively low
temperature. Other examples are the Helium-Mercury (HeHg) and Helium-Selenium
(HeSe) lasers, which are among those that can be built by a determined
amateur. See the chapter: Home-Built
Helium-Mercury (HeHg) and Other He-Metal-Vapor Lasers for more information.

The typical HeCd laser can produce a high quality beam at 442 nm (violet-blue)
and/or 325 nm (UV) depending on the optics. Typical power output is in the
10s to 100s of milliwatts range. In terms of popularity, the HeCd laser
probably ranks behind HeNe, Ar/Kr ion, and CO2 gas lasers. Although its
wavelengths may be highly desirable for some forms of spectroscopy,
non-destructive testing, and stereo lithography, they are pretty useless
for laser shows and other common hobbyist applications. For that
reason, as well as the higher complexity (and cost), one doesn't see
these lasers nearly as often as the more common types.

I'm aware of only two major manufacturers of HeCd lasers in the USA:
Melles Griot and
Kimmon. Melles Griot acquired the
HeCd laser companies Omnichrome and Liconix, and continues to offer both,
claiming to be the largest manufacturer of HeCd lasers. But
some people consider Kimmon to be the "gold standard" of HeCd lasers,
and they do have a wider selection of models, some with higher maximum power.

While most HeCd lasers operate at 325 nm and/or 442 nm,
one manufacturer, The Cooke
Corporation used to have a "white light" HeCd laser putting out 5 lines
in the red, green, and blue, out of a possible 12 lines with their design.
Except for its limited power (50 mW total max), such a laser would be good
for laser shows and laser TV. (As of 2006, perhaps earlier, this laser
appears to be discontinued but used units may be available. An unusual
laser like this will probably be a collectable someday.)

In fact, it's sometimes possible to get a few of these visible lines, at least
momentarily during warmup, from a normal UV (325 nm) HeCd laser and
this might seem quite surprising:

(From: Steve Roberts (osteven@akrobiz.com).)

"You just passed through the correct pressure for multiline operation
for a very brief period of time, and swept through the HeCd lasing
spectrum. Ours used to do that during warmup for about a half second.
I bet the OC is coated to reflect visible nearly 100% to keep visible
collateral (non-lasing) emissions to a minimum and it lased for just a
little bit on that. The pressure for UV and blue gain are quite far
apart, but not that far."

HeCd laser tubes ARE more complex than those used for HeNe, Ar/Kr ion, and CO2
lasers. In addition to often using a heated filament/cathode, they also
include a reservoir for the cadmium metal and a heater to control its vapor
pressure, a mechanism to add helium as needed to maintain correct pressure,
possibly an overall heater and thermal insulation to control tube temperature,
and various sensors inside the envelope to monitor these parameters for use
by several feedback loops in the power supply. The power supplies are also
correspondingly more complex with multiple feedback loops and power
sequencing logic.

As with other gas lasers, both internal and external mirror HeCd lasers
are available. External mirrors provide a polarized beam (due to the Brewster
windows) and allow for swapping of optics to to select 325 nm and/or 442 nm
operation. But for applications where this isn't important, internal mirrors
eliminate the need for optics cleaning. Even if the mirrors are internal,
they are often (screw) adjustable to some extent. Melles Griot purchased
the HeCd operations of both Omnichrome (internal mirror) and Liconix
(external mirror), and continue to manufacture HeCd lasers designed
by both companies. A few Omnichrome HeCd lasers are linearly polarized, I
assume through the use of a Brewster plate inside the tube.

The output power of HeCd lasers is generally fixed by operating conditions,
but by adjusting Cd pressure, may be traded off against optical noise. There
is usually no light feedback to regulate output power, though some Liconix
(and possibly other) HeCd lasers implement active noise reduction, which
probably uses optical feedback to control tube current. But since both
He and Cd are replenished as needed, the output power of a HeCd laser
should be fairly constant over its life. However, not adhering to the correct
power sequencing particularly during shutdown (accidental plug-pull or
power failure) can result in cadmium vapor condensing on the Brewster windows
or internal mirrors. Each event will result in a slight loss of performance
which can eventually add up to a very sick laser for which there may be no
cure. Omnichrome specifically warns about this while Liconix claims it
doesn't matter. While these HeCd lasers do use very different designs, I
would still be rather suspect of the claims.

Finding a working HeCd laser surplus at a reasonable price seems to be
virtually impossible. Most HeCd laser heads on eBay are in a very
poor state of health or totally dead. There's no way to easily
inspect or test a HeCd laser head or tube by itself for anything
beyond ionization. A HeCd laser tube depleted of cadmium (which is
what happens once it's operating hours have been used up) is basically
a high tech wall sculpture filled with a hazardous material (cadmium).
And by that time, they actually look quite ugly with cadmium debris
coating the interior of the tube in random places. High mileage HeNe laser
tube bores filled with brown crud look pristine by comparison. However,
an ugly tube may still work perfectly.

So, the only way to know for sure is to run the laser on the proper power
supply. Unless your electronics skills are quite sophisticated, it only
makes sense to use the correct mating power supply. So, finding a HeCd
laser head without power supply may not be such a great find. Substituting a
power supply from another manufacturer would also be challenging
because they may use different tube currents and incompatible schemes
for He and Cd pressure regulation. Building a power supply from
scratch is much more complex than for most other gas lasers. And,
close isn't good enough because there is a rather narrow range of
conditions for tube current, He pressure, and Cd pressure, where there
will be any lasing at all. Using trial and to find the correct
operating conditions may damage the tube beyond repair. I don't know
of any practical way to determine operating parameters experimentally
without fancy optical and electronic test instruments. Those sorts of
details are not forthcoming from HeCd laser companies.

Although the HeCd is still a gas laser, its construction is quite complex
compared to, say, a common HeNe laser tube. Specific reference may be
made to Omnichrome (Omni, now Melles Griot) HeCd lasers:

Gas fill: The largest portion of the gas fill is helium
(essentially 100 percent at startup) which sustains the electrical discharge
and excites the cadmium vapor, which is what actually is the lasing medium.
Overall helium pressure is regulated via closed loop feedback.
There may be a helium reserve with helium added as needed to maintain
correct pressure. Unlike a HeNe laser where there is no need to
replenish helium over the life of the laser, a HeCd
laser must periodically add helium to maintain the correct pressure.

The helium reservoir (with heater) and thermistor pressure/temperature
sensors in Omni HeCd lasers are located near the cathode-end of the tube.
There are two sets of sensors, each set sharing a common electrode, should
one set go bad. If one should go bad (not sure how this might happen!), it's
may be best to switch the wiring entirely to the other set, though any
two thermistors would probably work just about as well since they are all
identical. The partial pressure of helium is order of 1,000 times that
of cadmium during operation, so these pressure sensors are not affected
significantly by the cadmium pressure.

Bore (capillary): Like HeNe lasers, HeCd laser tubes have a
thick-walled tube with a small hole in it where the main discharge and
laser gain actually takes place. But in HeCd lasers, this may be made of
fused silica or quartz instead of glass due to the higher current and
bore temperature. The bore of Omni HeCd
lasers extends about an inch beyond where the Cd vapor enters the bore. The
discharge current through this "cataphoresis bore" assures that Cd vapor
only travels down the bore to the cadmium condenser near the cathode, and
not to the area of the HR (anode-end) mirror where it could condense.

Cadmium supply: In addition to the gas fill of helium, there
is a cadmium reservoir and wrap-around resistance
heater to maintain a specific (closed loop controlled) Cd vapor pressure in
the tube. At startup, the cadmium is at ambient temperature with negligible
vapor pressure. A minute or two after the heater is energized, cadmium
pressure begins to increase noticeably. Feedback may be based on tube
voltage or simply Cd temperature. During warmup, residual cadmium remaining
in the bore from previous operation may vaporize and momentarily produce
some lasing, which then disappears until the cadmium reservoir begins to
approach operating temperature. One of the factors determining HeCd tube
lifetime is how long enough Cd remains in the reservoir. The actual location
of the Cd reservoir may vary, but the Cd vapor is fed into the anode-end of
the tube. So, it may be near the anode, in a side-arm connected to the
anode, or somewhere else.

The Cd reservoir in Omni HeCd lasers surrounds the bore near the anode-end
of the bore (but not at or beyond it). A gap in the bore proveds access
for the Cd vapor.

Cadmium condenser: Once the cadmium has done its job in the bore,
it must be directed to end up in a place where it will not cause trouble.
A mirror or Brewster window would be bad. So, there is an area just beyond
the end of the bore for this purpose. It will generally be kept cool and
may have a magnet to help direct the Cd. Since there's no guarantee that
the Cd build up in an nice thin film on the wall of the tube, some lasers
have a "remelt" function that applies current to a heater surrounding the
cadmium condenser to melt any stalagmites that my have been formed which
could potentially block the beam in the bore.

Some longer Omni HeCd tubes have the remelt heater and a user activated
remelt function. Shorter ones do not but perhaps a heat gun could
be used with care to do this if needed. But the melting point of Cd is
about 321 °C. So, maybe not. :( :) However, Cd may sublimate before
it melts at low pressure, but I have no information on that.

Cathode: Since HeCd tubes operate at relatively high current - at
least compared to Helium-Neon (HeNe) lasers - a heated filament/cathode is
often used rather than the coaxial cold cathode design of a HeNe laser.

Anode: The positive electrode may be the mirror mount (internal
mirror tubes) or a separate electrode. WARNING: Potentially lethal voltage!

Omni HeCd laser tubes have the HR mirror mount flange as the anode and
there may be no WARNING stickers with respect to the high voltage dangers.
Removing the cooling fan at the HR-end of cylindrical HeCd lasers exposes
this with no protection.

Mirrors: HeCd laser tubes often have an internal mirror for just
the High Reflector (HR) or for the Output Coupler (OC) mirror as well. The
former arrangement allows line wavelength selection by changing an external
OC mirror or via a line selecting prism. Even with internal mirrors, some
adjustment is normally provided via compliant mirror mounts with accessible
screws. As usual, where external mirrors are used, Brewster windows seal
the end(s) of the tube.

Omni HeCd lasers use internal mirrors. Cylindrical Omni HeCd laser heads
use tubes with flanges that permit some mirror adjustment but they
are not attached to any part of the case or a resonator structure.
Rectangular Omni HeCd laser heads use tubes of similar design, but
the flanges are attached to a three-bar Invar resonator,
with large nuts for adjustment accessible from outside the case.

Temperature sensor(s): To prevent damage in the event of
overheating, one or more sensors on the tube. Omni tubes have a silicon
diode for this purpose. If the forward voltage drop is greater than 0.5 V,
the temperature is low enough for starting. Below 0.25 V, the laser will
shut down.

WARNING: Since the temperature of the tube affects helium and cadmium pressure,
both critical for proper operation, major parts of the tube may be covered with
thermal insulation. At a minimum, this will surround the He and Cd heaters.
In older lasers, THE MATERIAL WAS ASBESTOS! Thus, HAZMAT handling
procedures apply any time maintenance or modifications are being done inside
the laser head!

CAUTION: If covers or shrouds are removed to gain access for testing,
or just to admire the pretty glow, behavior may be different due to the change
in the thermal environment. Inadequate cooling may also be bad for the tube.

This is one big Helium Cadmium Laser tube, measuring 32.5" in length. It is
made for the typical 3 rod resonator frame. Note the typical bright
gold/yellow color of the discharge with no pink from air leakage visible.

This tube goes in the model 374 laser head which is a larger version of
the 456 head. The head's model number indicates the lasing wavelength:
A 374 is 325 nm (near-UV) and a 474 or 456 is the 442 nm (violet). Omnichrome
is now a part of Melles Griot. Lasers essentially similar to the original
Omnichrome line are still being manufactured, though a new and improved
power supply (the LC500) has replaced the Omni-100 we all know and love. :)

For each model series (e.g., X39), the number is the distance between the
mirrors in cm. All are normal single bore tubes except for the X112 which
is basically two X56 tubes joined end-to-end and powered separately.

The actual operating voltage set-point may be varied over a 6 to 7 percent
range via the R2 pot on the laser head PCB. The controlled variable is
actually Cd temperature but the reference is tube voltage, which is a strong
function of Cd pressure. This may be used to optimize power or noise
performance.

In some ways, the HeCd laser has the hazards of both larger HeNe lasers
and ion lasers.

As with *any* laser, proper precautions must be taken to avoid any
possibility of damage to vision. The power output of a HeCd laser may be
10s to 100s of mW continuous at one or both of two wavelengths. The
brightness fo the 442 nm (violet-blue) is deceptive because of the human
eye's low sensitivity to this wavelength - the perceived brightness is only
about 1/25th that at 555 nm (green) or about 1/6th that of the common red
HeNe (632.8 nm, actually appears orange-red) laser. However, the potential
for damage depends on the optical power, not the perceived brightness! The
325 nm (UV) output is, of course, totally invisible and therefore even more
dangerous.

The power supply for the HeCd laser operates at between 700 and 3,000 VDC
with 100 mA or more of tube current. This is a quite lethal combination.
With HeNe power supplies where the maximum current is under 10 mA, while you
may receive an annoying or even painful shock and drop the tube, you will
likely live through it (may not apply to gross overkill home-built power
supplies!). However, with HeCd power supplies, there is much greater
potential (no pun...) for death due to the higher available current.
The HR mirror mount of Omnichrome (now Melles Griot) tubes is the anode.

Portions of the HeCd laser tube may be hot enough to burn flesh in
addition to being electrically live.

Cadmium is classified as a toxic heavy metal. This isn't a problem with an
intact laser tube. However, should the tube be broken, don't attempt to
salvage anything unless you have appropriate experience in dealing with these
substances and avoid the temptation to purchase (or even inherit) a HeCd
laser with a broken tube. In addition, as noted above, asbestos may be
present on older lasers for use as thermal insulation. Where you have no
choice, wet down the asbestos, seal everything in a thick plastic bag, and
dispose of it properly.

Helium-Cadmium Laser Testing, Adjustment, Repair

The typical HeCd laser isn't all that different from other gas lasers - a tube
that can leak or break, HV power supply, circuit boards and cabling that can
develop bad connections, etc. The main additional items found in a HeCd laser
are the heated cadmium reservoir (with a finite supply of cadmium) and its
temperature regulator, and some type of system for maintaining helium
pressure. Lasing will only occur within a narrow range of both Cd and He
conditions so both their control loops must be functioning properly for
lasing to occur. And, maximum output power and minimum optical noise
require even more precise conditions.

The cadmium doesn't get used up, combine with anything else, or leave the
tube, but as the laser is operated, the cadmium vapor is whisked down the
tube from anode to cathode in a process called cataphoresis and is deposited
(hopefully) in a special area for this purpose called the cadmium condenser.
However, some will also end up on the walls of the tube and elsewhere - away
from the heater. Once all of it's gone from the reservoir, there is no easy
way to get it back (though I can think of some not-so-easy ways that might
work). The rate of cadmium depletion is on the order of 1 mg/hour. A typical
cadmium reservoir is originally loaded with a few grams of cadmium. So,
that is ultimately what limits the life of any HeCd laser.

The helium doesn't get used up either, but some of it ends up being buried
under the deposited cadmium and on/in the electrodes. This reduces the
pressure in the tube as it is run so there is usually some means
of replenishing helium from a helium reservoir. Omnichrome (now Melles
Griot) HeCd lasers have a sophisticated He pressure sensing system for
this purpose. Liconix HeCd may maintain Cd temperature constant and then use
tube voltage for sensing helium pressure.

Unlike a HeNe laser tube where power tends to peak and then decline
with use since it's sealed with a fixed quantity of helium and neon gas,
a HeCd laser in principle can operate near full power and then die within
a few minutes when the last bit of cadmium is vaporized. Until then, both
He and Cd are replenished as needed. (It's not likely that lack of He will
be a problem since there is orders of magnitude excess.) However, as a
practical matter, there will be other mechanisms reducing output power over
their life. Nothing is perfectly pure and contaminants
are outgassed as the laser runs. Cd on the bore walls reduces mode
volume. Ion damage to internal mirror tubes and Cd on Brewster windows
or mirrors all contribute. So, as with other lasers, there's usually
a "warranty" power which will be lower than the typical output power
when new. And although some HeCd lasers may run for 10,000 hours or more,
the typical life expectancy is in the 4,000 to 6,000 hour range.

Given the added complexity and very specific operating conditions,
the cause of problems with an otherwise undamaged HeCd laser even if
it lights up is much more likely to be electronic in nature than with, say,
a common HeNe laser. If any of the control loops isn't functioning
properly, there probably will not be any beam.

The additional complexity of a HeCd laser may at first seem overwhelming
but there are really only 4 things that determine whether a given otherwise
healthy HeCd laser will lase. The terms "otherwise healthy" mean that the
tube intact with no significant gas leakage and the Brewster windows and/or
mirrors are in good condition.

Tube current: The manufacturer will specify a tube current or
current range. While not super critical, it's certainly worth confirming
proper current to eliminate it as a possible cause of a low or no lasing
condition.

Helium pressure: There should be some way of determining the
correct value, usually indirectly by measuring tube voltage or the voltage
across a heat loss sensing thermistor. Too low or too high and there will
be no lasing. Slightly low and the laser may go through a period of weak
lasing as the cadmium pressure increases and then die out.

Cadmium pressure: There should be some way of determining the
correct value, usually indirectly by measuring tube voltage (at the correct
He pressure) or by the temperature of the Cd heater. If the He pressure is
correct, there will be a relatively wide range over which some lasing will
take place, but only a narrow range where the output power will be maximized
and/or there will be minimum noise.

Mirror alignment: And, finally, of course, the mirrors need to be
quite well aligned for these narrow bore lasers. This is best checked with
the tube off using an external alignment HeNe laser or an autocollimator.

Here are some common problems and possible causes. In general, for
diagnosing a laser with not output beam,
avoid the temptation to fiddle with the mirror alignment unless it's known
that someone has messed it up! Confirm proper operation of the He and
Cd pressure control loops with electrical tests and/or viewing of the
discharge spectrum. Specific reference may be made to the Omni-100 power
supply but most of the info should apply with obvious modifications to
other types.

Tube will not start:

You forgot to plug in the power supply or turn it on. :)

No high voltage.

Test with HV probe in power supply before starter, or at power supply
or laser head test points.

No start voltage.

Test with HV probe or by arcing to 1M ohm high voltage resistor to ground.

WARNING: 27 kV or more may be present if tube does not start.

CAUTION: DO NOT allow the power supply to continue trying to start for
more than a few seconds. Apparently, the Omni-100 design is such that
it may die after not too long.

CAUTION: DO NOT bypass the interlock with the HV cable unplugged from the
power supply unless it can hold off 27 kV. The round AMP connector on the
Omni-100 and Omni laser HeCd heads definitely cannot without the mating
connector plugged in and fully seated. If the starting voltage is present,
it will probably then arc between the pins which will damage the connector
and possibly components inside the power supply, though it should shut down
before electrical failure occurs. The plastic barriers in the cable-end AMP
connector must be present to provide enough insulation, and even this appears
to be marginal. Why didn't they use something more like a 4 pin Alden
connector? :)

Tube high pressure or up to air.

Disconnect tube from power supply and test for ionization with Tesla/Oudin
coil, low power RF source, or HeNe laser power supply.

Filament (if present) not lit.

Test for filament voltage. Test filament for continuity.

High voltage cable arcing or electrically leaky. Symptom is that the
Omni-100 shuts down with the red "Over Current" fault LED lit as it tries
to start the tube. This LED is located on the top PCB on the right side
and is visible through the grill. Fortunately, the Omni-100
seems to tolerate these faults well without blowing up.

Inspect the contacts inside both the cable and power supply and laser head
connectors. This is a likely place for arcing, especially if the HV
connector wasn't fully inserted and tightened at one or both ends. (Or
you tried to bypass the HV interlock to test for HV without the cable
or laser head being plugged as noted above!)
This connector type is very marginal for the
up to 27 kV that may be present during starting. A slight amount of damage
may be cleaned up with alcohol and degreaser and such a repair might last
awhile. Replacement of the damaged connector(s) is best.

There may be leakage, arcing, or a short inside the cable as well. Check
with an ohmmeter and/or HiPot tester. If a problem is found, while a cut
and splice job might be possible if the bad section can be located,
replacement of the entire cable is the best solution.

Tube starts but there is no output:

Beam shutter closed. :)

You didn't wait long enough for warmup. :)

It's working great but is a UV (325 nm) laser. :)

See the section on testing for UV.

Tube current is incorrect.

Measure tube current directly, or at power supply or laser head test points.

Cd pressure is incorrect.

Cd pressure low: If the discharge color never changes from its initial
appearance of yellow-white (helium only), check that the power to heater
is at near maximum, and if that's confirmed, check heater continuity. If
they are OK, inspect the Cd reservoir (after allowing the tube to cool!)
visually by peeling back the thermal insulation surrounding it.

Cd pressure high: If the discharge color passes through the normal
white operation region and then goes to "gulping" blue, check for correct
functioning of the feedback circuitry, either voltage (Omni-type) or
temperature (Liconix-type). If the tube voltage is correct (Omni-type),
then He pressure may be high? If the tube voltage is too low (either
type), the feedback isn't working.

He pressure is incorrect.

Not sure yet how to test this directly beyond making sure for the Omni-type
that the sensor thermistors have the correct resistance at room temperature
(5K to 6K ohms) and that the comparator and drive circuitry are working
correctly.

Cd contamination on mirror(s) or Brewster window(s).

Where the discharge color and tube voltage are normal, inspect the mirrors
for contamination. Some evidence of contamination may be detected by
shining a red HeNe laser beam through the mirror and looking for excessive
scatter on the inner surface. There should be virtually no scatter.

Mirrors are out of alignment.

Remove the cover over the end with the output mirror. Allow the laser
to warm up and then gently press on the flange or mount at several
locations around its perimeter. If there is a very slight misalignment,
this may result in some lasing, and a starting point for adjusting the
alignment. DON'T touch the mirror alignment adjustment screws UNLESS
it is already lasing! In that case, NEVER lose lasing entirely!

WARNING: On Omnichrome/Melles Griot HeCd lasers, the HR-end mirror mount
flange has the full high voltage on it. This is potentially very lethal.
Testing at the output-end - which is perfectly safe - should be sufficient
to determine if mirror alignment is the problem.

Output appears during warmup but goes away:

You didn't wait long enough for warmup. The beam may reappear. :)

Cd pressure is too high.

Check the color of the discharge. If it is blue indicating "gulping",
check the tube voltage. If it is normal, He pressure may be incorrect.
If it is low, there is a fault with the voltage regulation circuitry
(Omni-type) or Cd temperature regulation circuitry (Liconix-type).

He pressure is too low.

Check the He pressure sensors (if any), He heater, and He regulation
circuitry.

The is written with respect to the Omnichrome 439-3 laser head but should
apply in general:

(From: R. J. Zimmerman (safed_chuha@yahoo.com).)

Omnichrome HeCd lasers have a heated filament/cathode. Near the output-end
of the laser are three leads on metal feed-throughs in the glass. These
lead to two filaments. You can measure if they are open circuit with a
multimeter.

If they are both open circuit (with the center pole between them) then the
cathode is dead. Still, the laser would probably start if it weren't up
to air. This would kill it very quickly from a condition that Omnichrome
called "cold cathode." The plasma would search for electrons anywhere it could
and eventually find them on the nickel ion shield on the inside of the
laser just before the output mirror. In the process of stripping out
electrons, nickel gets sputtered all over the inside of the glass near the
output and it looks much like an old burnt out vacuum tube with a mirrored
finish. The mirror gets coated too and this condition which is very rare,
destroys the tube as there is no reasonable way to replace the optic.

Of course opening the glass would also require you to reprocess the laser
and backfill it with a near vacuum of Helium, so it will be hard to find
someone to do this indeed.

While the 442 nm (violet-blue) output of a HeCd laser will only appear about
1/6th as bright as a red (632.8 nm) HeNe laser for the same output power), it
is quite visible. However, many systems are set up for single line output at
325 nm which is well into the UV portion of the spectrum and most definitely
invisible. This wavelength is actually on the border between UV-A and UV-B so
perhaps those black-light fluorescence mineral fanatics may know the best
way of detecting it. :)

However, it really is very easy. Almost any common white paper stock will
fluoresce blue quite nicely. Orange "Handle with Care" or "Danger" stickers
fluoresce bright yellow. In fact, it's interesting and fun to see what type of
fluorescence the HeCd 325 nm beam will produce with many common materials.

(From: Don Klipstein (Don@Misty.com).)

I have found that white cotton underwear and white cotton socks fluoresce blue
from wavelengths as low as the 253.7 nm mercury line and as high as a scandium
line that I believe is 391 nm. Works on everything in between such as the 313
nm and 365 to 366 nm line clusters of high pressure mercury. That's my best
bet for 325 nm.

I doubt that most fluorescent lamp phosphors would work. But the phosphor
from a high pressure mercury lamp is supposed to work from 313 nm and I give
it a good chance at 325 nm. Most mercury lamp phosphors don't work at 365 nm
but one does at least somewhat - from the oddball and probably obsolete
Westinghouse "Standard White".

(From: Steve Roberts (osteven@akrobiz.com).)

Hey, we have a UV HeCd laser here at the University. I just tweaked the
system it's in. However, I learned the hard way about UV HeCds - the UV they
pump out is kind of hard to detect and at 13 mW was swamped on my fluorescent
paper by the bore light. I had to eat some humble pie when after proclaiming
that it wasn't lasing. But, if you move the paper way out from the laser, the
beam is there and the bore light isn't. The beam diameter was very large (1.1
mm) compared to your average HeNe or air-cooled argon. Trying a bunch of
different fluorescent materials resulted in disappointment as very few seemed
to be pumped well by the 325 nm wavelength. Over the years I've been
contacted by many people who said they had surplus Hecds that didn't lase,
even with low hours, no matter what they did. I have a funny feeling quite a
few of those were actually UV machines. It does show up on the power meter
however.

I once had a long positive column self heated HeCd laser where the gain curve
was centered around one temperature and fell off to near zero 5 degrees C
either way from that point. That point varied from tube to tube, but was not
mentioned in the manual, only the fact that if you didn't have the
temperature spot on, you had no gain. Modern units are designed for a
slightly wider operating range and usually have a glass frit and a
molecular sieve to adjust the helium pressure as well. So don't dispare if
you don't see any lasing for a while, HeCd lasers are difficult and the PSU may
have to be matched to a given laser. You're not just looking for the Cd
melting point, you're looking for the place where it has a high vapor
pressure, which may be considerably hotter. Ask the manufacturer, they
sometimes will tell what you need on a obsolete model, especially if you're
a student.

As with other gas lasers, the color of the discharge, particularly in the
narrow bore, can provide valuable diagnostic info. For rectangular laser
heads, simply removing one of the cover screws may provide a small hole
to view the bore. This is preferable to removing the entire cover, which
will affect the thermal environment. For cylindrical laser heads, it may
be necessary to carefully drill a hole through the aluminum if there are
no mounting screw or ventilation grill holes in a suitable location.
Put a collar on your drill bit to prevent drilling through the glass tube
inside (!!) and minimize vibrations when drilling! For example,
the bore is unobstructed by insulation or any other covering about 11 inches
from the output-end of the Omni laser heads I've seen but this should
be confirmed before drilling the hole if possible. A small hole drilled
in that area provides a very nice display of the discharge.

Normal startup: Pure helium yellow-ish-white. If observing with
a spectrascope or diffraction grating, there is a very intense yellow line,
a less intense red line, and a spread of blue and green lines.

Normal operation: Fairly neutral white. Blue, green, and red
cadmium lines will be present. The cadmium red line in particular will
become quite intense and distinct from the He red line.

All HeCd laser tubes provide a mechanism to replenish helium that gets buried
under the condensed cadmium vapor, is adsorbed on the electrodes, and leaks
out of the glass walls of the tube (however slowly that may be).

But the only practical way to reduce helium pressure without opening and
repumping the tube is by letting it run for a long long time. This could
be months or years. There may be some scavenging of He if run without
heating the cadmium, but it's probably at a much slower rate.
So there will be a tradeoff in running with Cd being heated and high
Cd pressure which should accelerate the rate of reduction of He pressure,
but will use up more Cd in the process. I do not know whether there is
an optimal Cd temperature that results in being left with the most Cd
when all is said and done, but doesn't take totally forever.

Melles Griot claims that Omni-type tubes do not need to be run periodically
to keep them healthy. But people who have used HeCd lasers tend to disagree.
Maybe the technology has improved and there is no significant rate of
diffusion of He from the high pressure He reservoir. Since modern tubes
are hard-sealed, this is the only likely issue with non-use. Of course,
if the power supply control loop for the He pressure was defective and let
loose a pile of He, there may be no hope at all before the Cd is depleted.

The amount of usable cadmium in the Cd reservoir is what ultimately
limits the life of a HeCd laser. The Cd reserve is typically about
5 grams when new and the discharge transfers about 1 mg of Cd per
hour down the bore of the tube by cataphoresis under normal operating
conditions. So the life expectancy of a HeCd laser
is on the order of 4,000 to 5,000 hours. HeCd lasers should be run in
"Standby" mode (if available) when not actually being used for more
than 1/2 hour or so. Standby reduces the plasma tube current and
turns off the Cd heater entirely. The time to return to normal
operation is approximately cut in half compared to a cold start.
With the Cd heater off, no Cd is being used during Standby.

There is no easy way to convince the Cd to go back to its reservoir It
has been suggested that this might be accomplished by keeping the Cd
reservoir cold and using a heat gun to evaporate the Cd in other parts of
the tube so it will go there. However, when these approaches are evaluated
more carefully, they don't sound very plausible, except possibly as an
experiment. Normally, I'd be all for that, but if the tube should shatter
during the attempt, there will be toxic cadmium all over the place.

There are a few problems. The melting point of Cd is about
321 °C (610 °F) and the boiling point is about 767 °C
(1407 °F) at 1 atm. The vapor pressure is only 1 Torr at 400 °C and
16 Torr at 500 °C. But, under low pressure, Cd may sublimate - go
directly from solid to gas - even before it melts. I have no information
on that except for a NASA note that prohibits the use of Cd in a hard vacuum
since sublimation may occur above temperatures as low as 75 °C. So,
sublimation provides a possible mechanism to move the Cd, but it could be
a very slow process. If sublimation doesn't help, trying to heat large
parts of the tube to a temperature hotter than the clean cycle on a domestic
self cleaning oven - approaching cherry red - and keeping the Cd reservoir
cool at the same time without cracking the tube or destroying the optics
would be quite challenging. All external thermal insulation, He and Cd
heaters, wiring, and sensors would have to be removed. Whether internal
mirror frit seals or Brewster windows would survive is another issue.
And to further complicate things, the path to the Cd reservoir is through
the narrow bore. In keeping the reservoir cold, there will be other
parts of the tube with easier access that will also be as cold or colder.
Getting a few mg of Cd back there might be possible, but a useful quantity
would be another story. Another problem with any cadmium redistribution
scheme is that any helium that was buried under the condensed cadmium
during normal operation - and this is probably a good portion of what was
replaced from the He supply - would be released, resulting in excessive
He pressure and no way to reduce it without running a loooong time and
using up the Cd. :) So, this approach really doesn't sound plausible.
Comments to the contrary or otherwise are welcome.

I really don't know why they don't provide more cadmium. It's not expensive.
Sure, other parts of the system might fail, but running out of Cd is
guaranteed to result in a dead laser. But, redundant filaments and He
temperature/pressure sensors are provided in Omni HeCd tubes. I guess
they would sell fewer lasers if they lasted longer. :)

Helium-Cadmium Laser Power Supplies

Filament supply - Many of these HeCd tubes use a heated
filament for the cathode (unlike modern HeNe lasers which use a cold cathode).
This is usually just a regulated low voltage DC source. Omnichrome (now
Melles Griot) HeCd laser tubes are constructed like this. Older Liconix
HeCd lasers may use a cold cathode neon sign-like electrode.

High voltage DC - A source of 700 to 2,000 VDC or more (depending
on the particular tube length and bore diameter). Like a HeNe tube, the
HeCd tube is a negative resistance device and requires a current regulated
power supply. Typical current is in the 10s to 100s of mA. Commercial HeCd
laser power supplies normally use switchmode/inverter type designs with
controllers using pulse width modulation. Like other gas lasers, a higher
starting voltage is also needed, up to 10 kV or more. This is often
provided by a voltage multiplier.

Helium heater - A heating element wrapped around a high pressure He flask
is used to adjust overall helium pressure by releasing He through a
membrane that passes helium when heated (upward only). The noraml He pressure
is a few Torr.

Cadmium reservoir heater - This provides a means of maintaining a constant
partial pressure of cadmium vapor in the tube (along with the permanent helium
fill). The normal operating pressure for Cd vapor is a few milliTorr.

At least two types of schemes are used to control He and Cd in HeCd lasers.

Omnichrome (now Melles Griot) HeCd lasers control He pressure directly
using thermistor sensors inside the tube and control Cd pressure based on
tube voltage.

Liconix (also now Melles Griot) may operate at constant Cd heater
temperature with tube voltage used to sense He pressure.

The Liconix approach is slightly simpler but may be more sensitive to ambient
conditions. I do not know what approach other companies like Kimmon may use.

The following control loops are provided in Omnichrome HeCd lasers. These
are electrically independent, though there will be some interaction based
on changes in He and Cd pressure.

Tube discharge current control loop: - Maintains a constant current
through the tube once it has started. For a typical Omnichrome HeCd laser,
this is 100 mA during normal operation and 82 mA during Standby and Shutdown
(until it is turned off entirely by the shutdown timer).

Helium pressure control loop: - A pair of thermistors sense both He
temperature and pressure in a bridge configuration. One thermistor (RT1)
is operated at a low current with little heating and thus responds only to
ambient temperature inside the tube. The other thermistor (RT2) is operated
at a higher current that results in significant heating and its temperature
is increased above ambient inside the tube. But heat loss is a function of
helium pressure so the actual temperature will be lower when the helium
pressure is higher. A resistor network (to equalize the voltaegs of RT1
and RT2 when in equilibrium) feeds an LM311 comparator which outputs a HIGH
level when helium pressure needs to be increased. This turns on a triac
to heat the helium reservoir temperature permiable membrane
until helium pressure has increased to the set-point level. A small amount
of hysteresis in the LM311 feedback circuit assures that the heater is
only on or off. (The pressure when new is about 3 atm, and given the
rate at which helium is needed, this really doesn't change with use.)
The membrane is actually a thin piece of glass which when at a high
temperature, passes helium at a slow, but adequate rate, to provided
what is needed even if it does take hours.

The time constant for the He control loop is about an *hour*. And the effect
of requesting more helium takes several hours to occur. So, even
though the He heater may come on for a few minutes when the laser starts up,
this has essentially no effect on He pressure. It probably occurs simply
because the pressure is lower than normal until the tube reaches operating
temperature.

Since the normal helium pressure is a few Torr, compared to a few milliTorr
for the cadmium pressure, the He pressure is what primarily determines the
overall gas pressure inside the tube. In addition, the area where the He
pressure sensors are located doesn't see much of the Cd vapor as it's
condensed before reaching there.

Cadmium pressure control loop: - With the tube started, voltage
across the discharge at a given current is a strong function of Cd pressure.
An increase in Cd pressure results in a lower tube voltage. A small heater
is used on the Cd reservoir to adjust Cd pressure to achieve the specified
tube voltage for optimal operation. The actual tube voltage is compared
to the set-point value with the PID error signal controlling the phase angle
for firing a triac that drives the Cd heater.

The time constant for the Cd control loop is about 30 seconds with Cd pressure
stabilization requiring a few minutes. At startup, the Cd heater will be on
at its maximum power (about 80 percent duty cycle for the Omni-100 power
supply). This will descrease to 35 or 40 percent after warmup (lower if
the ambient temperature is higher and vice-versa).

Note: The Cd Heater LED on the front panel of the Omni-100 only pulses at
60 Hz (every other triac cycle) so it's hard to estimate duty cycle by
looking at it with unaided Mark-1 eyeballs!

Timing and control logic assures correct startup and shutdown sequencing,
which is critical for long life. Fault detection circuitry monitors tube
temperature to shutdown the system or prevent a restart if it becomes
excessive (due to a fan failure or blocked air-flow). It is critical to
the life of Omni HeCd lasers to adhere to the rules for shutdown. A primary
reason for this is to assure that the Cd doesn't condense where it shouldn't
be, particularly on the mirrors or Brewster windows. If power were simply shut
off while the Cd heater was on, there would be Cd vapor in the tube, with
some more coming from the Cd reservoir before it cooled off. But, without
the discharge to provide some direction to the Cd, it would simply flow to
all parts of the tube and some would condense on the optics. It wouldn't
take too many of these "events" to kill the laser entirely. For Omni HeCd
lasers, The discharge MUST be maintained until the cadmium reservoir has
had time to cool down.

Some/most/all Liconix and Kimmon HeCd lasers have no power down sequencing and
can simply be shut off due to the tube structure and thermal design with
heatsinks surrounding the tube at strategic locations to condense Cd before it
can get to the Brewster windows or mirrors. However, optics contamination
requiring tube replacement has been a common problem with HeCd lasers
in general so perhaps this doesn't always work reliably.

The following discussion applies directly to the Omnichrome HeCd power
supplies. I don't know how relevant they are for Liconix and others.

(From: Daniel Ames (dlames3@msn.com).)

Ya, these are some really weird and overly complicated animals. I have spent
probably a hundred hours or more trying to diagnose my non-working HeCd PSUs
which are still not working.

Power output is lower than expected:

Are you sure it is dying? How does the cadmium supply in the reservoir look?
You should be able to see the (silver to aluminum in color) Cd metal from
either end of the reservoir without having to remove its heater and covering.
What color is the side discharge of the tube (gold, white, or blue)?

Is the power supply the right one for this tube? These MDL100-A's can be set
up to run the larger Omnichrome series 74 heads also by changing the two large
resistors near the fan in the PSU and readjusting the settings.

The He and or Cd heater control circuit may be out of adjustment. Also
the reference voltage and current adjust trim pots could possibly not be
optimized for your tube and power supply. From what I understand, they need to
be tuned to each other to work their best, not just any MDL100 on
any 56 head. It could still work, but could cause either a shortened tube
life or low power output.

I would NOT recommend trying to adjust any of the electronics as they ALL
interact with each other to allow the tube to operate correctly. 0r
incorrectly!

NOTE: If you are going to try adjusting anything, trimpots or optical
alignment, don't try to adjust anything without first making two reference
marks, one for the stationary position and one on the item being adjusted,
i.e., optical alignment nuts and/or the trimpot adjust slots. Plus, you would
need to know what the ball-park settings should be. I do not remember what they
should be set at, sorry.

Could the optic's alignment need a little fine tunning? Yikes, for UV you
will definitely need a power meter!

Maybe you can just apply some physical pressure in different directions
(after the tube is warmed up and stabilized) to the resonator rods separately,
or on the mirror plates and watch the power meter. (WARNING: Take care with
respect to the high voltage at the HR end!) If the output power increases,
then the optical alignment needs some fine tuning.

And, for the low output power: Did you measure this output power with a
properly calibrated power meter that is designed for this wavelength? If you
used some other power meter, or a generic meter, most likely its response will
not be accurate at the violet-blue or especially the UV wavelength. Most
likely it will read lower than the actual output. If you know what type of
photodiode it uses, then if you can find the optical response specs for it,
you will be able to figure out the response % or ratio for the 442 or 325 nm
wavelengths (or the combination if running multiline).

Fan doesn't run:

Is this the fan in the laser head or the power supply?

The filaments come on: They may be burnt out or just not operating due
to bad connections or a bad component in the power supply. The filament
regulator is fairly simple and it's easy enough to check the filaments and
wiring for continuity.

Filament lights up but the discharge will not come on:

There is probably no high voltage, and/or start voltage. see below.

The high voltage cable arcs inside:

I hate it when this happens. :(

WARNING: From experience, DO NOT TRY TO USE THIS HV CABLE, or you could
damage the electronics in both the power supply and the head!

Unable to locate a replacement for the SG3527J driver chip:

Right... Silicon General was bought out by
Unitrode (Now Texas Instruments) but they discontinued this chip or it was
extinct before they took
over SG's line and did not bring this chip back. Are you sure it is not an
SG3627J (not that this helps much)? All my MDL100As use this chip for the HV
control chip. The SG3627J and its interchangeable cousins, SG1627 and SG2627,
became extinct back in 1991 and Omnichrome no longer has them in stock, but
they do make an after market circuit board that will substitute for it. Last
time I checked (approximately a year ago), it cost about $70.00. Yikes!!!!
I hear that there is a way around this, but I am not sure of how it is done.
I can't find a datasheet for the SG1627, but the
SG1549 Datasheet
does have a functional block diagram (on the last page).

The high voltage transistors (MJ10007) cross reference to a NTE97. The
SG1524/SG2524/SG3524 cross reference's to a NTE1720. Note: These 3 SG chips
are the same chip but with different operating temperature ranges, either
will work just fine with these PSUs.

(From: Sam.)

I believe the SG3627 is a multiple channel MOSFET driver so this would imply
that some sort of transistor network can be installed in its place (for each
channel). This shouldn't be that difficult to implement or find a substitute
part (though not pin compatible).

The SG1524/SG2524/SG3524 is still available for less than $1 from
Mouser and other suppliers. I expect that
and original MJ10007 replacement can be found as well. (ECG/NTE/SK parts can
be used in a pinch but may be of variable quality and are almost always much
more expensive than the real thing).

(From: Daniel.)

I have now found a distributor that most likely still has some SG1627 Chips for
Omnichrome HeCd (and other) switchmode PSUs. I had contacted SGS Thomson
through their web site, who referred me to a distributor, so I contacted them
back in March, 1999, and they had 97+ of these chips. Omnichrome used many of
the SG1627, SG2627, and SG3627 for the same chip. However this distributor does
have a minimum order, (*&%$##*) which at that time was $100.00. The more you
(or we together spend) the more chips for the dollar we get.
Contact: Ron Holmes (ronh@accgrp.com).

High voltage is not present:

Many different things can cause the high voltage DC to not work or shut down:

The entire HV control circuit and any of the several bridge rectifiers on
the board may be defective.

Any or all of the three HV transistors could be bad and/or their fuses
could be blown.

The HV high frequency transformer and the voltage multiplier might be
defective.

The high voltage cable or the plugs and sockets could have developed a
carbonized short.

One of the SSRs (Solid State Relays) could be bad.

Many other related components could be bad.

The tube could be up to air (much higher than normal gas pressure).
This can be tested with a hand-held Tesla (Oudin) coil or HeNe laser
power supply.

Additional Information on HeCd Lasers

The following applies specifically to Omnichrome X39, X56/X056, and X074
HeCd laser heads with Omni-100/A/B power supplies. (There is also an X112
series which is basically a pair of X056 tubes joined end-to-end with two
Omni-100 power supplies. They must both be switched on and off at the
same time using their front panel switches, or via the remote control.)

Specifications for all the models in current production can be found on the
Melles Griot Web site under
"Product Info", "Lasers", "HeCd".

The Omni-100/A power supply is for the X39, X56/X056, and X112 lasers while
the Omni-100B power supply is for the X074 laser. The value of the internal
ballast resistors in the Omni-100 and Omni-100B power supplies is supposed
to be different, but I don't know if anything else, like the HV inverter
circuitry, also differs. I don't know if there is any functional difference
between the Omni-100 and Omni-100A or if the latter is simply a later (and
current) model.

An Omni-156, which was used with some versions of the X56 laser head,
also exists. I don't know whether it came before or after the
Omni-100, or if other model-specific variations were built.
But the Omni-156 appears functionally identical to the Omni-100s
except for the lack of Standby mode. The construction is similar
with most of the same components in the
same locations, but the LEDs on the front panel are arranged horizontally
instead of vertically. :) A toggle switch substitutes for the 3 position
rotary switch found on the Omni-100s. There is also one other known
difference: The HV power and interlock connections on the 4 pin high
voltage laser head cable are swapped compared to the Omni-100s. So,
attempting to run a normal X56 laser head with this supply, or vice-versa,
should result in absolutely nothing happening! I don't believe one or both
will explode though. I discovered then when I went through the checklist
of resistance tests on one of the funky 456 laser heads. This
supposedly assures that the power supply won't be damaged by a
defective laser head. Everything passed except the high voltage
cable and there was no evidence that it had been modified. I later
acquired an Omni-156 power supply, which had the swapped wiring.

CAUTION: The Omni-100/A/B power supplies are NOT interchangeable with
X39, X56/X056, and X074 laser heads! Attempting to run an X074 head on
an Omni-100 will likely blow the power supply upon power-on. Although
the connectors are the same with the same pinouts, inverter components
will blow as the high voltage ramps up but the tube doesn't light. I
would imagine the opposite situation to be no better, though I don't
know what happens.

Melles Griot no longer seems to manufacture the cylindrical Omnichrome HeCd
laser heads. The rectangular ones all have 4 digits in their model numbers -
X056, X074, and X112. The really short X39 seems to have disappeared
entirely.

I have been testing non-lasing Omni model 439-5 laser head with an Omni-100
power supply. More about this specific laser below.
I also have a healthy Omni model 3074 UV HeCd laser. The info below
is partially from my observations and partially from the Omni operation
manual.

CAUTION: Before applying power, make sure all three circular connectors are
fully seated and tight. Failure to do this with the high voltage connector -
which is already marginal for the voltage it may need to handle during
starting - may result in arcing inside and destruction of both the cable-end
and power supply connectors. Fortunately, the Omni-100 power supply
seems to tolerate these faults well without blowing up. The red "Over
Current" fault LED (located inside on the top PCB on the right side
and visible through the grill) will come on as it tries to start the
tube. The green Power LED will remain lit, but the power supply is
essentially shut down and nothing else will happen until AC power is
removed entirely for at least 3 seconds.

Basic operation of the Omni-100 power supply

The power up sequence should go something like:

Apply AC power and switch the rotary knob to "Operate". The power LED on
the power supply and the red LED on the laser head come on; the fans
in both the power supply and laser head spin up.

Within a few seconds, the discharge comes on a yellow-white color (in the
bore).

The Cd Heater LED come on.

The He Heater LED may come on for a minute or so but generally shouldn't
stay on once the laser has warmed up. If it just flashes or doesn't
come on at all, the control circuit may be defective or the He pressure
may be way too high. But the lack of the He LED coming on doesn't
necessarily mean something is broken as long as the laser is otherwise
operating normally.

Once the Cd is at operating temperature (sensed by tube voltage drop)
the Cd heater LED will go off and on or run at low duty cycle to maintain
regulation. A laser beam should be present once this happens, if it
wasn't already. (It may have come and gone once or twice.)

When switched to "Shutdown", the Cd and He Heater LEDs go off and the discharge
should become very slightly dimmer since it's current is reduced (from about
100 mA down to 82 mA). After 2 minutes and 17 seconds, the discharge will go
out and the power supply fan will turn off. 35 seconds later, all power will
shut off. During this latter 35 seconds, the red Temperature Lockout LED will
probably come on indicating that the tube is too hot to restart. (Delay times
are with respect to 60 Hz line frequency. On 50 Hz, they will be 20 percent
longer.) The system will not restart even if switched back to
Opreate or Standby until the Temperature Lockout LED goes off in another
few minutes, but will do so automatically once it does.

When switched to Standby, the discharge is at 82 mA but the Cd heater is
off so there will be no beam after the Cd in the bore is depleted. Running
in Standby when not actually being used reduces the time to get to full
power compared to a cold start. I also recommend switching to Standby
for 10 minutes or so before switching to Shutdown as the relatively
short time delay in Shutdown before all power is removed may not be
sufficient to guarantee that all the Cd vapor is gone from the tube.

Melles Griot claims you don't have to run these lasers periodically to keep
them healthy, at least not on newer lasers. However, it may be better to
do so to be safe, perhaps a couple hours a month.

Checking or monitoring the high voltage

On newer Omni HeCd laser heads, there are externally accessible
test points which include one for the high voltage. On older heads, the
cover over the front of the head must be removed to gain access but it's
much safer to check voltages there than in the power supply since only
low voltages are present there. The high voltage
to the tube (before the starter) may be determined by measuring across R19,
the resistor that personalizes the laser head to the power supply in terms
of tube voltage. The actual voltage will then be V*10M/R19. R19=21.0K for
X39 tubes, 13.3K for X56 tubes, and 12.1K for X112 tubes. I don't know what
the value of R19 is for X74 tubes but it's probably between 8K and 10K.

I constructed a little widget which is now wired across R19 in my 439-5 laser
head with a high resistance voltage divider so that my trusty Radio Shack DMM
displays the voltage directly.

Fine tuning cadmium temperature:

R2 is the fine adjustment for the tube voltage set-point based on cadmium
pressure. The range of R2 is about 7 percent or 0.7 percent/turn.
For testing or diagnostic purposes, an increased range may be desirable without
changing the original set-point. The cadmium temperature can be tweaked
externally by paralleling a resistor with R19 (to reduce it) or by injecting
a small current via a resistor and DC power supply (to raise it).

CAUTION: DO NOT even think about touching R18, the helium pressure
set-point!!! I don't know of an adjustment procedure and if you set it
incorrectly such that the helium pressure goes too high, there is no way to
get it back down without running the laser for a looooong time. Put a
piece of tape over the pot to avoid temptation or accidents!

Mirror alignment:

WARNING: The HR-end mirror mount of Omni HeCd lasers has the full high
voltage and is potentially very lethal. If the fan at the HR-end of
cylindrical laser heads, or the cover of rectangular laser heads is
removed, the HV is exposed. DO NOT remove the fan or attempt to adjust
mirror alignment on the non-output (HR) end of cylindrical Omni HeCd lasers!
The screws and flange are electrically live with up to 2,000 V or more!
Mirror alignment screws for rectangular Omni HeCd laser heads are accessible
through 3 holes at each end of the case and are safe. All you can do is
mess up lasing or possibly crack the tube if they are turned too far.
Mirror alignment isn't the problem unless someone before you messed with it!

Omni HeCd lasers have three screws or nuts at each end for mirror adjustment.

On cylindrical heads, the screws press on a flange which slightly distorts
something like a single flap metal bellows. Removing a cover at the OC-end
enables access to its mirror adjustment screws. As noted above, the mirror
mount at the HR-end of the head is electrically live and extremely dangerous
so don't go there with power on!

On rectangular laser heads, the mirror mounts are supported by a three
bar Invar resonator frame with nuts accessible through holes in the case
(usually covered with plastic plugs. Both ends are isolated from the
high voltage and a nut driver or socket wrench can be used to turn them.

Like any other laser, fine tuning of mirror alignment including mirror walking
can be performed as long as there is some lasing at all. If there is no beam
but everything else about the laser checks out (tube voltage, discharge color,
etc.), then mirror alignment may be the problem, though only likely if
someone before you attempted to adjust it. It's not likely to change on
its own.

A HeCd laser is similar to other narrow-bore gas lasers in terms of suitable
alignment techniques. For the Omni HeCd lasers, the closest is probably
internal mirror helium-neon laser alignment, with the understanding that
the actual adjustments can be performed using the screws or nuts and no
metal bending is needed. :) Coarse alignment (unpowered) can be performed
using a red HeNe laser beam aligned to cleanly pass through the HeCd bore,
adjusting the reflections from both mirrors to coincide with the HeNe
laser's output aperture. Or, an autocollimator can be used. Then peak and
walk the mirrors once there is some lasing.

This Standby voltage was measured just after switching from Operate to
Standby after running for awhile after stabilizing in Operate.

This Operate voltage was measured just after switching from Standby to
Operate after reaching equilibrium in Standby.

These data were taken at the laser head test points. Note that the tube
shows a positive resistance in this case. I do not know if the negative
resistance of the sick laser is a symptom or whether the difference is due
to measuring the voltage before or after the ballast resistors.

I also found something interesting which I first noticed after leaving
Operate and switching to Standby. The laser continued to emit a weak beam
well after it should have ceased according to the manual. And, when
powering up in Standby, the same weak beam appeared fairly quickly, before
the Cd should have had a chance to be heated sufficiently for normal lasing.
In Standby, no Cd heating is supposed to be taking place and the Cd Heater
LED on the Omni-100 was always off. However, since the Cd Heater LED only
monitors 1/2 of the AC cycle, it's possible for the Cd to still be heated
at a reduced power level due to a failure of the triac that controls the
Cd heater in the Omni-100 power supply. The triac could be turning on
for the other half cycle with no indication on the front panel, though such
a triac failure mode is quite unusual and would almost certainly result in an
inability to regulate Cd pressure since the minimum duty cycle would be over
50 percent. More likely, this quirk
is caused by some residual Cd in the bore or near the bore
that is getting heated by the discharge. In fact, after about 1/2 hour
in Standby, lasing ceased entirely and the tube voltage climbed to a stable
2,940 V, presumably as the Cd was depleted from where it shouldn't have been.
I will be keeping an eye on this behavior in the future. I'm not sure of
the implications should this occur frequently during Shutdown with some
Cd vapor still present when the system actually turns off.

The tube voltage versus time charts for Operate and Standby
after the Cd had been fully cleaned out of the bore or whereever, are
shown below:

I'm not sure of the cause of the small maxima and minima in tube voltage
during normal startup but they do occur consistently. They are not present
with the sick Omni-459 discussed in the next section.

The time required for lasing to cease when entering Standby seems to depend
on how long the laser had been running in Operate. If run for only a couple
minutes after stabilizing, lasing will cease in under 3 minutes and possibly
by the 2 minute and 17 second time when the discharge would be turned of
during Shutdown. However, when run for about 20 minutes, as was the case
above, lasing continued for almost 6 minutes, though the output power did
decrease to below 10 percent of the operating power by the 2 minute mark.
I do not know what the limiting value for the "cease lasing" time is if the
laser is run continuously for days. Based on the condition the laser was
in when I acquired it assuming it had been run continuously in its former
life, that time was much greater than 10 minutes, with the time until the
tube voltage leveled off measured in hours (but that final tail may be
unrelated).

Since lasing may cease entirely only well after the normal delay of 2
minutes and 17 seconds in Shutoff (which is identical to Standby except for
actually turning off the system), I'm wondering if it would be better to run
in Standby for several minutes - until several minutes beyond the time when
there is no output beam - and then switch to Shutdown, rather than going to
Shutdown directly from Operate. Any lasing suggests that some Cd vapor is
still present at that time and beyond, though perhaps it's guaranteed to be
so small by then to be of no consequence. But even if this is the case,
using the extended shutdown procedure won't hurt and may be better for the
long term health of the laser.

As part of the effort to determine the problem with the HeCd laser described
in the next section I used a monochromator to look at the discharge spectrum.
The chart below shows all the lines found in the discharge between
approximately 400 and 700 nm that were more than 1 or 2 percent of the most
intense helium line at 587.6 nm. There was no convenient way to position the
entrance slit of the monochromator next to the bore so it is actually looking
through a hole in the case that normally allows access to the tube voltage
adjust pot. :) It is more or less in the center of the laser so I assume most
of the light comes from the positive column in the bore:

Note how the intensities of all the He lines decrease when the Cd is present.

The wavelengths were only approximate from the monochromator (+/-1 nm perhaps)
but were checked against the
NIST
Atomic Spectra Database for Cd and He to confirm their source and
determine their exact wavelength.

I acquired this Omni model 459-5 HeCd cylindrical laser head along with a
dead Omni-100 power supply, and an apparently healthy Omni-100 power supply
separately. It is the latter that I'm using for testing. One reason I'm
optimistic about this laser head is that although it is quite old (1985)
and hasn't been used in many years, the time meter only shows about
200 hours. It may have been taken out of service due to the dead power
supply. But when I attempted to unsolder the time meter from the head PCB
to gain easier access to the terminals, one lead fell off. So, perhaps,
the time meter just hasn't worked since the 200 hour mark!

In any case, there is one major problem and one minor problem:

Incorrect tube voltage characteristics and no lasing:

OK, maybe that's two major problems. :)

The laser starts without hessitation and runs normally, but the tube
voltage doesn't behave anything like it should according to the Omni
operation manual and there is no output at any time, though the
discharge color at startup looks correct in comparison with a working
Omni-3074 HeCd laser. The discharge color then passes through a period
where it would be correct for normal operation, but due to the incorrect
tube voltage, finally stabilizes with excessive cadmium temperature, a
blue discharge, and "gulping". From startup until beyond the normal
operation point, the spectra look similar (at least by eye using a
diffraction grating) with the cadmium red line appearing as the Cd
temperature rises and becoming quite intense.

At 950 V, the discharge color is way to blue ("gulping") meaning that it
takes much higher Cd vapor pressure to reduce the tube voltage to the
set-point value.

When the voltage is measured with the discharge color similar to the
working Omni-3074, the tube voltage is closer to 1100 V.

Note the negative resistance of the tube - about -1.33K ohms. This value
could probably provide some diagnostic information as well if I only knew
what! :)

Of course, the actual voltages are quite different for the Omni-3074 and
Omni-439, but it's quite clear that with the healthy laser, the vaporization
of the cadmium has a much more dramatic effect on tube voltage compared
to the small change only from tube warmup (without the cadmium being heated).

By 1 minute after entering Standby, there is virtually no Cd vapor remaining
in the bore.

For now, I've paralleled R19 (the resistor in the laser head that determines
the coarse tube voltage set-point based on tube type) with 120K ohms to make
the Omni-100 control loop stabilize at a cadmium temperature that produces
approximately the correct discharge color. In essence, the controller thinks
the tube voltage is really 950 V and is very happy. Hopefully, this will
bury helium in the condensed cadmium and drive down the helium pressure
until conditions are suitable for lasing without using an excessive amount
of cadmium. I also removed the He heater fuse (F7) to make doubly sure that
the power supply doesn't get any wild ideas about adding helium. :)

Measurements made on the helium pressure sensing circuitry are not promising.
Here are the voltages found on the terminals of the laser head PCB in the
Omni-439-5:

* For pins 5 and 9, the actual tube voltage is found by multiplying these
values by 496.16 (10,000,000/R19).

I added a cable from the laser head PCB so that the temperature and pressure
sensor voltages (on RT1 and RT2), in addition to the measured tube voltage
(on R19) could be conveniently monitored since these cylindrical laser heads
have no external test points.

The two thermistor sensors (RT1 and RT2) are close together at the cathode-end
of the tube near the helium reservoir. They both should have a resistance of
around 5K ohms at 25 °C.

RT1 is fed a very low current (about 1 mA) that results in almost
no heating of RT1 and therefore it monitors the temperature inside the tube
(actually very close to ambient) but is not appreciably affected by helium
pressure.

For this laser, the voltage across RT1 is 5.26 V resulting in a voltage, V1,
of 5.88 V at the noninverting input of the helium heater comparator, AR1, on
the laser head PCB (an LM311 used with a 1M resistor for positive feedback to
provide a small amount of hysteresis). (These is a resistor network to
adjust the calibration accounting for the difference in RT1 voltage and
comparator voltage.) And the resistance of RT1 is very close to 5K ohms so
it is at about 25 °C.

RT2 is fed a higher current so that it dissipates enough power for its
temperature to rise significantly. It is also affected by ambient
temperature in the same way as RT1, but how high its temperature goes above
this also depends on the heat transfer to the helium, and that's a strong
function of He pressure. Higher pressure means more heat transfer and a lower
RT2 temperature.

For this laser, the current through RT2 is 3.46 mA, its resistance is
2.433K ohms, and the voltage, V2, across it and into the inverting
input of AR1 is 8.46 V.

With V1 being less than V2 - much much less for this laser - the He heater is
solidly off. There are only two possible explanations: (1) one or both
thermistors are defective or (2) the helium pressure is much too high. A
higher He pressure results in more heat transfer from the thermistor and a
lower temperature, lower current, and higher resistance. In fact for the two
inputs of the comparator to be equal, the resistance of RT2 would have to
be only 494 ohms. Indeed, paralleling RT2 with a 470 ohm resistor turns on
the He Heater LED. RT1 being around 5K ohms seems perfectly reasonable
but RT2's values are - not to put too fine a point on it - messed up.
To rule out RT2, I tested both of the spare thermistors with an external
circuit and they behaved exactly the same as RT2. The heat transfer is
so large that its temperature doesn't increase by more than a few °C.
So the He pressure is probably very high and/or there is gas contamination.
The pressure can probably be determined analytically, though I wouldn't want
to try! But it certainly would appear to be very high and I rather doubt
that any sort of treatment that doesn't involve reprocessing the tube will
help - even running it for a year!

Note that the expected currents in both RT1 and RT2 differ by about a factor of
3 compared to the description in the Omni operation manual. That references
3 mA and 30 mA for RT1 and RT2, respectively. Based on the schematics and
the actual circuitry in this laser head, the corresponding values at the
point of equilibrium would be 1 mA and 12 mA. The larger current are
essentially impossible under any conditions. I bet the description is for
an older version of the laser and it was never fully updated.

Next, in an effort to determine if there was gas contamination, I
used my monochromator to look at the discharge spectrum through that hole
I drilled in the cylindrical laser head. The chart below
shows the spectra of the sick Omni-459-5 next to the healthy Omni-3074,
with the data normalized to make the strong 587.6 nm He lines of equal
intensity in Standby:

The sick laser was run in Operate mode with a tube voltage that resulted in
approximately the correct discharge color. With a healthy HeCd laser,
the He line at 643.8 nm and the Cd line at 667.8 nm should appear of
about equal intensity and they do by eye. While there is a significant
difference in the measured intensities, it's actually less than in
the healthy laser. And in any case, the adjustment of the discharge
color was only approximate. I also cranked up the Cd and run it gulping
for a few minutes, and indeed the intensities of the Cd lines I checked
increased by several fold.

It is of significance that there are no detectable lines likely from other
sources than He and Cd. Anything with a high enough concentration to adversely
affect lasing would probably have been detected unless it was very close to a
He or Cd line and both were measured as a single peak.

While there are differences between intensities of the lines in the two lasers,
what's amazing is how similar they really are for the most part. Some of the
discrepancy can be attributed to measurement error as the monochromator was
simply sitting next to the lasers. For the Omni-3074 in particular, the gain
was cranked way up since it was viewing the discharge indirectly through a
small hole, so any movement would result in a large change in signal level.
Certainly, except perhaps to an experienced spectroscopist, nothing jumps out
and says: "I'm the problem!". :) I guess this is both good and bad.

And finally, I checked mirror alignment using a red HeNe laser bouncing off
both the OC mirror and through the bore off the HR mirror. Pressing on
the flanges of each mirror confirmed the source of the reflections. The
alignment was fine and the return beams didn't show excessive scatter
that might indicate degradation or contamination of the mirrors.

So, in summary:

The tube voltage is wrong - it starts out low (around 1,200 V instead of
1,300 V) but requires more Cd to drop to around the spec'd operating
voltage (950 V), and by then is gulping badly.

Measurements of the helium pressure sensing circuitry are consistent
with very excessive pressure since the RT2 thermistor resistance is too
high by a factor of almost 5.

The only significant spectral lines come from helium and cadmium so there
can't be much, if any, gas contamination. The lines present and their
intensities is similar to those of a healthy Omni-3074 HeCd laser. This
includes the ratio of the intensities of the He line at 643.8 nm and the
Cd line at 667.8 nm which is reasonable for normal operation.

The mirror alignment seems acceptable and the mirrors appear to be in good
condition.

There is never any lasing, not a single coherent photon at 442 nm!

Therefore, the conclusion remains that the He pressure is simply way too high
and this is preventing lasing.

Intermittent Cd heater connection:

This occasionally causes the Cd to not start heating until the head is tapped
or the cables are jiggled. There were no obvious bad connections at the
accessible cathode-end of the head cylinder, though a pair of splices could
not be inspected. Getting to the anode-end would require pulling the tube.

The only problem if it's in the heater itself is replacing the coating of
thermal insulation over it after the repair. What is that stuff? It's not
asbestos. Does it come in spray cans? :) If wet down, it falls apart. The
entire heater is just some thin strips of nichrome or steal wrapped around the
tube and spot welded together. Maybe NASA can help. They know all about
thermal insulation!

But given the high helium pressure, an intermittent Cd heater may only be
an academic curiosity and the repair would be straitforward in any case. :( :)

Open Questions

Is all this consistent with high helium pressure? Everything
else about the tube appears normal but no lasing. Mirror alignment has not
been touched but it has been unused for years.

How high does helium pressure have to be before lasing ceases entirely?

What is the relationship of helium pressure to tube voltage? Everyone
seems to think high pressure means higher voltage but that's opposite
of the behavior of this laser.

If I'm going to need to run it for a month or year :) to get
the He pressure down, does this need to be done in Operate mode which uses
up Cd, or will Standby mode work (though perhaps slower)? Given how far
off it is, would running have any chance of helping? What is the rate of
depletion of helium?

How does the Omni-100A differ from the Omni-100 or Omni-100B
power supplies?

As noted elsewhere, HeCd lasers with internal helium sources
have a tendency to go over-pressure after sitting for some time. In
Omnichrome heads, this results in "gulping" as the tube voltage-controlled
cadmium heater works overtime to lower the voltage. Almost
all Omnichrome lasers made before 1990 seem to have some degree of this
malady, but there is hope without reprocessing the tube. One effect of
high helium pressure is that the tube will not lase at the original
operating current. Often though, lowering the operating current (along
with adjustment of the cadmium heater controls) will put you back into
a lasing regime. So far, all three of mine have responded favorably to
this. A quick test you can try: From a cold start, switch the laser
to Operate and watch the discharge color. As it starts to become
white, but before serious gulping begins, switch to Standby. Now, it may very
well lase for a while as you are sweeping the cadmium-to-helium ratio
at reduced current. At the 82 mA standby current, these tubes will
still function, so you can go ahead and modify settings as you need.
If this test fails, more dramatic means might be necessary to recover
lasing.

Liconix HeCd lasers are of a very different design than those from Omnichrome.
Some photos of Liconix HeCd lasers can be found in the
Laser Equipment Gallery.
under "Liconix Helium-Cadmium Lasers".

They use a cold cathode which looks like an oversize neon sign electrode and
the anode is a wire electrode next to the Cd reservoir. The control loops
are constant temperature to regulate Cd pressure and tube voltage to regulate
He pressure. There is a large gas reservoir connected via a side-arm, with the
high pressure He reservoir surrounded by its heater at the far end. Almost
all of the tube is covered in thermal insulation including the entire bore.
The laser heads use convection cooling - no fan(s). Partially for this reason,
their operating temperature range is somewhat smaller than Omni.

All Liconix HeCd lasers I know of have external mirrors and a Brewster window
tube so the output is linearly polarized and the mirrors may be
interchangeable for 325 nm, 442 nm, or both wavelengths at the same time.

I'm not sure of the startup in newer Liconix HeCd lasers, but on older ones,
turning the key-lock power switch
to the run position will cause the power supply to attempt to start the tube
immediately. If the tube does not strike, or does not stabilize with the
proper operating current, the starting voltage will be applied for a few
seconds, then cut off, then is applied again in second or so length pulses
about 15 times. If the tube has still not started successfully, it then
gives up. I'm not sure if it tries again later but a restart
can be forced by grounding the lone pin in the center of the power supply
control PCB. (More on this coming.) A healthy laser should start instantly
and stay on with a solid bright yellow helium discharge color. One that's
sat idle for just a bit too long may require a few restarts to stay lit and
then should be run for a good long time to make it happy again. If it's too
far gone, the starting process will not work and during that time, the
discharge - if it lights at all - will be erratic and stuttering. The
discharge color may be pink or purple near the cathodes if there is
significant gas contamination. It should be honey-yellow to yellow-white
throughout. And none of this will help. Most Liconix
lasers found on eBay are too far gone. In some cases, it may be possible
to run them for a few days or weeks or months on a HeNe laser power supply
if they won't stay lit with their own power supply to clean up the
contamination, but that's a long shot.

Unlike Omnichrome, Liconix HeCd lasers have no shutdown sequencing
requirements. They supposedly use lower tube current and have an advanced
cold trap design so Cd vapor condensing on the Brewster windows is not a
problem. This better work since there is no equivalent to Omnichorme's
tandby mode for the Liconix power supplies.

Higher power Liconix HeCd lasers use a tube with two bores.
For these, the neon sign electrode cathodes with the negative high
voltage on them are electrically at each end (physically in side-arms) fed
through external ballast resistors. The Cd reservoir with its heater, and
anode terminal are in the center near ground potential. The large gas
reservoir, and helium reservoir and heater, are attached as a side-arm
near one end. The power supply for the dual bore lasers has two complete
and almost totally independent HV sections with common control circuitry.

I've heard of using an Oudin coil to help start these lasers where the
discharge doesn't want to go down the bore, but rather seems to pulse
through the gas reservoir. However, use this with caution as I've also
heard that the power supply (or at least the starter) might be damaged
by this stunt, or it might have just been a coincidence where one output
of a dual output power supply failed when attempting to restart after
having run the laser for several hours. I'll have more on this if info
becomes available.

Apparently, Liconix HeCd laser tubes are all soft-sealed so periodically
running them is essential to long life. Leave them alone and they may
end up being quite dead after a few years. I don't know if this has improved
with current production models.

Specifications for all the models in current production can be found on the
Melles Griot Web site under
"Product Info", "Lasers", "HeCd".

I'm not actually absolutely sure what make or model HeCd lasers the following
applies to, but it's NOT Omni:

(From: Mike Hager (mhager@sbcglobal.net).)

You can check for the Cadmium by using a transmission grating and
watching the discharge from the middle of the capillary tube. You will
have a blue, a couple of green, a yellow and one red line. After about
two minutes, you will see more blue, green lines, and the important
second red cadmium line.

A small hole should be good enough to see the plasma inside the capillary, if
not remove some of the insulation so you can view the discharge.

The He heater should come on only very infrequently or never. For a one
minute shot of heating the He reservoir, you will need around 100 hours of run
time to deplete it. Most of the time, the He heater LED may only come on at
start up, if then. This should only last a second or two until the control
circuit stabilizes.

This is not the case of the cadmium partial pressure which will deplete
in minutes! HeCd tubes can be hard to start if the He pressure is too high,
sometimes using crushed dry ice around the tube will drop it enough to
ionize the gas. The only problem is condensation on the tube. I completely
disconnect the He heater so there is no chance for a shot unless it
really needs it.

The Cd heater circuit is a fixed temperature loop, not relying on tube
pressure/voltage for control. However the He does use a diode in addition to
monitoring tube voltage for its control. If the discharge color is changing
from a pinkish to blue, then you have a condition known as "gulping". This
is caused by too much cadmium heater voltage, monitor the test points and
reduce until no more blue is noticed.

It is suggested to run these HeCd lasers for 24 hours for every month of
storage. The reason is that the the Helium will diffuse from its high
pressure reservoir into the tube, resulting in a high He pressure/tube
voltage. I have had to run some tubes for months to bring the He
pressure down far enough to allow lasing. You can't just raise the Cd
partial pressure to get these things to lase. Keep monitoring the tube
voltage day after day and you will see a drop from the depletion of
excess He.

If the OC mirror has a yellow tint (in transmission), it's a 441.6 nm head.
If it is clear with a slight hue on it, it is a 325 nm UV head.

The following applies to Liconix 4000 series HeCd lasers including the
4207, 4210, and the dual discharge 4240. The symptoms are that
the laser will start but then shut off in a few seconds.

(Portions from: Phil Bergeron (pbergero@cas.usf.edu).)

WARNING: The high voltage to the tube can be quite deadly. You must have a
healthy respect for power supplies producing several kV at 90 mA! You should
see the sparks to ground they can throw. Think of what that will do to a nice
moist human body. Unplug the supply from the wall and disconnect the HV
wires from the laser head before touching anything inside and
keep hands away from the laser tube when it is on!

IMPORTANT: Before applying power again, remove the ballast resistor covers
in the laser head to avoid a spark-over to ground. That is critical since
with a hard-starting tube, the voltage may go high enough to jump the
undersize gap, and that may eventually take out the control chip(s).

I'd also recommend disconnecting the helium heater to prevent any possibility
of the power supply deciding incorrectly that He is needed, making matters
worse.

The cause of starting problems is that the He pressure is way too high so the
sustaining voltage is out of range of the supply until some Cd vapor is
present. So we added a simple push button switch to defeat the start timer.
Bypass the start timer and let it try to start. If it winks out, immediately
restart it. Melles Griot recommends to try for about a minute or so and then
wait a few minutes before another attempt (presumably to allow the power
supply to rest). Once it stays on long enough for some Cd to vaporize, the
tube voltage will come down and it should stay on.

Then let the lsaer run continuously to drive down the He pressure - for 2 to 3
weeks or maybe longer. No kidding! I revived a laser like this only to have
some unnamed faculty member ignore it for too long that it is now quite dead.